The central retinal projection in mammals has three components; the primary optic tracts terminate in the geniculate nuclei, pretectal area and tectum, the accessory optic tracts terminate in the terminal nuclei of the accessory optic system, and the retinohypothalamic tracts terminate in the suprachiasmatic nuclei of the medial hypothalamus. The latter projection was only clearly defined during the last three years of this project (prior title "A Comparative Study of Tectothalamic Connections") and these observations have led to a change in the research program from largely anatomical studies to a combined functional-anatomical approach. The purpose of the program now is both to analyze the morphology of the retinohypothalamic projection and its terminal nuclei and to determine the functional role of the projection in the regulation of diurnal rhythms. Anatomical studies will include a developmental study of the retinohypothalamic projection and the suprachiasmatic nuclei, an analysis of afferent and efferent connections of the suprachiasmatic nuclei using both conventional techniques (Golgi, Fink-Heimer) and newly-developed techniques depending upon anterograde or retrograde axonal transport. Functional studies will analyze the effects of lesions in selected areas determined by the anatomical studies on diurnal rhythms in pineal function and pituitary function. The objective of the studies is to provide a map of central structures which participate in the neural control of diurnal rhythms.